Angelus was free. He was finally free of that awful soul that held him back so long. He was finally free to torture and kill all those who got in his way. Free to be himself finally after years and years of torture.
First, he decides to mess with the slayer while she still believes him to be her precious lap dog angel. So he walks back to Angel's apartment to see her waking up.
She says, "Angel where did you go?"
Angelus was so happy he got to be so cruel for the first time in a very long time. He says, "I was out hoping you'd leave before I got back. Look, you just don't have what it takes to please me. I am going out again don't be here when I get back."
She starts crying and she says, "Angel what's wrong with you?" She can't see the change in Angel. She can't tell that Angel no longer exists and that Angelus is in control.
He laughs as he says, "Buffy I was only with you for one thing and you gave it up a lot easier than I thought you would."
Angelus may be cruel, but he has been forced to watch Angel take all his choices away from him. Maybe he makes bad or evil choices, but they are his to make. Angel wanted to be human and refused to accept that he is a vampire and deal with what that means.
He laughs again as she cries harder and he walks out the door. He decides he would get something to eat before going to see his child and grand childe. He walked the dark streets of Sunnydale before he saw a lone woman walking.
He approaches her with a charming smile, "Hello miss."
She smiles back, but before she can say a word his face changes and she screams he covers her mouth and sinks his fangs into her throat and drops her when he's done. He didn't kill her, he didn't want to let people know he was back yet.
Time to go home to the family, he thinks as he heads off to the warehouse to see Spike and Drusilla. Sunnydale has no idea what's in store for it. Angelus can't wait to show them.